Free T4 index (T7)
What it measures, and what changes the number.
The free T4 index, also called T7, is a calculated estimate of the thyroxine that is not bound to proteins in the blood. It is obtained by combining a total T4 result with a T3 uptake (thyroid hormone binding ratio) result, which reflects how many hormone binding sites are available.
It is read with total T4, TSH and sometimes total T3. The index was designed to correct total T4 for changes in binding proteins, and it has largely been replaced by direct free T4 immunoassays in many laboratories.
Because it depends on two separate tests, the method used for each affects the result, and values from different labs may not be comparable. It is not the same as a directly measured free T4. Reports may print it as FREE T4 INDEX (T7), FTI or T7.